Chromium plating process



March 28, 1933.

W. M. PHILLIPS CHROMIUM 'PLATING PROCESS Filed Jan. 15, 1927 'Patented' Mu. 2s, 1933 UNITED STATES. PArENr OFFICE. i

'WILLIAI I., PHILLIPS, 0F BIBmGHAK, MICHIGAN, ASSIGNOB T0 GmBAL IOTOBS CORPORATION, 0F DETROIT, HICHIGAN', .A CORPORATION 0F DELAWARE einem rnarnm rnocEss Application led Imam 15, 1987. Serial No. 161,849.

The invention relates to electro-deposition "of metallic chromium especially for the production upon c nducting surfaces of 'a chromium plating for protective or decorative purposes. The effect of a layer of chro mium in resisting corrosion and in protecting metals coated 4therewith when exposed to atmospheric and other corrosive agencies is well known, also its hardness and resistance to wear. The principal object of the invention, therefore, is to provide means wherebyfchromium plating maybe carried on commercially with a high de ee of uniformity in results and excellence 1n the product, both in appearance and in utilization of the desirable properties of chromium plate. To this end the invention comprises novel features in the treatment of the plating bath and of the articles rto be plated, together with other details, all as herelnafter described and set forth in the appended claims.

In the accompanying drawing, Fi a vertical sectlonal view partly agremmatic showin a plating apparatus which maybe used 1n practicing the processes of the invention. Y

Fig. 2 is a section on line 2-2 of Figure 1, and u p j Fig. `3 is an enlarged sectional view through a portionaof a radiator shell for automobiles showin the arrangement of relis coatings on a typica commercial article.

Referring tothe drawing, 10 indicates a tank which is preferably formed of sheet steel and in which'is a plating bath 11. The tank is shown as provided with flanges 10' at its upper edge on which is supported a bus bar 12 insulated from the tank .by non-conducting blocks 13. Current nay be supplied from any suitable source, as generator 14 connected to the bus bar and tank respective- .l ly by conductors 15, 16. An article to be plated, as an automobile radiator shell 17,

maybe suspended in the bath by conducting hooks 18 engaging the bus bar. A coil 19, through -which heating .or cooling medium may be circulated, may be provided for maintaining the desired temperature of the,bath. Thechromium deposit can be. applied to various conducting surfaces upon which the quires no bufiing.

chromium surface may give beneficial results .as to 'appearance or durability, as upon the surface of articles of iron or Steel, nickel, brass, copper, etc. When a bright surface finish of the chromium coat is desired the surface to which it is ap lie may rst be polished or bulfed. A right coating of chromium can then be deposited which reto thervferrous articles and depositing chromium over the copper-nickel coating.

As shown in the drawing, the radiator shell 17 is composed of a pressed steel body portion 20 upon which are deposited successive layers V21, 22, 23 of copper, nickel and chromium, respectively.

` The preferred procedure is as follows:

The steel is rst coated with copper and nickelv in the usual manner. The nickel is bued as usual and then cleaned, preferably in an electrolytic alkaline bath in which the work is made the negative electrode. It is then Vrinsed inwater and dipped inan acid such as dilute sulphuric or muriatic, rinsed in Water and transferred to the chromium plating bath. The acid treatment removes an almost infinitesimal alkaline film, which, if al'- lowed to remain, often produces a cloudy effect inthe chromium plate. The work is left inthe chromium plating bath for a short time, preferably 1 to 5 minutes, and is then removed and washed in water. By this means a naturally lustrous coating 1s produced which needs no buiiing or olishing during its life, and which protects t e underlying metals from corrosion.

As shown, the tank 10 serves both as a container for the bath and as an anode for the supply of current. A material predominantly of ferrous metal, for example ordinary low carbon steel, is especially suitable for the ma- "By this means a brilliant 'fected by the ingredients of the bath which I employ for chromium plating and the tank or anode may be readily formed from sheet material.

The bath comprises soluble compounds of chromium principally chromic acid, CrOs, together with a relatively small amount of chromium sulphate, Cr2(SO4) 8, and preferably a small quantity of a compound containing the boric acid radical, for example, of boric acid. The proportions of the ingredients may be given as approximately 200 gr. of chromic acid, 1 to 12% gr. of chromium sulphate and 1 to 12% gr. of boric acid per liter of water.

' The chromic acid of the bath serves as a reservoir for the supply of the chromium which is ydeposited'electrolytically at the cathode. Itis necessary, therefore, to replenish the chromic acid at intervals in order to maintain the original concentration. The preferred compos1tion of a freshly prepared bath may be given as follows:

20g-2610 grams per liter of chromic acid, i. e.,

3-6 grams per liter of chromium sulphate i. e., orasog. i

8 grams per liter of boric acid, i. e., HBBOS It is important, for satisfactory operation of the bath, to restrict the total content of sulphates, including chromium sulphate, to that given, viz., not more than 121/2 gr. per liter, and preferably, for most uniform results, not more than approximately 6 gr. per liter. Since commercial grades of chromic acid usually contain some sulphates (including sulphuric acid) as impurities it will be obvious that the sulphate content of the bath will tend to increase with successive additions of chromic acid containing such impurities. It is essential therefore that the sulphate content of the chromic acid employed be limited to that which will not raise the total sulphate content beyond the desired range. A rela-- tively small sulphate content in the acid is not objectionable since it serves to replenish the sulphates in the bath which are removed by loss of solution. Y

It is also important that the content in the bath of ingredients tending to corrode the ferrous metal of thecontainer or anode preferably employed shall be restricted to an amount incapable of effecting undesirable corrosion. The ingredients specified for the bath have been found to have substantially no effect on the metal, but certain impurities usually present in commercial chromic acid, especially chlorides (including hydrochloric acid) tend to attack iron. It is desirable therefore, to restrict the content of such impurities in the chromic acid vemployed to the extent necessary to avoid their `accumulation beyond a safe limit.

It will be understood that the loss of solution by drag-out, i. e. by removal of solution adhering to the lated articles, and by spray, etc., will ten to keep down the undue accumulation of impurit1es. I have found it possible therefore to operate very successfully with chromic acid of ordinary commercial Grade with a specified puri of at least 987i. I prefer, however, to hmit the content of sulphates and of chlorides in the acid to approximately 0.5% of each. Such degree of purity has been found wholl satisfactory in factory production, althoug it will be understood that sli ht variations may be desirable above or be ow the limits set due `to the fact that the percentage of loss of solution will vary somewhat de ending upon the character of the articles eing plated and with the amount of chromium applied thereto.

In order `for the deposition of metallic chromium to take-place it is necessary that chromium in reduced form, such as compounds which can be identified by analysis for Cr20, be present in the bath, as it appears that deposition will not proceed satisfactorily when the chromium is present only as chromic acid and chromium sulphate. Therefore it may be desirable to treat a freshly prepared bath by passing current therethrough for--a short time before attempting to deposit chromium. This action causes reduction of suliicient chromium to provide for subsequent electrolytic deposition. The reduction, however, begins immediately with the flow of current when the anode area is greater than the cathode area, so that I have found it possible to begin platingas soon as the bath 1s prepared.

Since the reduction of chromium from the chromic acid continues during the deposition of metallic chromium there is a tendency for the bath to become continually richer in compounds of reduced chromium. Inpractice it is found advantageous for the CrzO, content to be kept below a value of approximately 25 gr. per liter. A certain degree of concentratlon may be reached in the normal operation of the bath at which an apparent state of approximate equilibrium occurs. At any time, however, when it is desirable to lower the proportion of reduced compounds it is only necessary to reverse the process by which the reduction is effected by substituting for the article bein lated an anode of relatively small area o s eet lead, for example, and passin the bath in a direction 4t used during deposition of metal and at a. current density below that necessary for deposition of metal, lthus reoxidizing a portion of the reduced chromium.

The bath is preferably maintained at a relatively high temperature during the platcurrent through e reverse of that ing operation as a much closer control of current density must be maintained if the temperature is low. In practice a temperature of 105 140 F. is most satisfactory. The current density is also kept at a relatively high point, the best results being secured with a current density -of 100 to 200 amperes per square foot at the cathode.

It will be understood that slight variations in the details set forth may be made by one skilled in the art without de arting from the scope of the invention an therefore I do not wish to be restricted to the specific features except as required by the language of th appended claims in view of the prior art.

claim:

1. The process of depositing chromium electrolytically comprising passing electric current through abath containing chromic acid with the use of a ferrous metal anode under conditions tending to produce inthe bath an excessive amount of reduced chromium, and at intervals electrolyzing the bath with another anode of different material and under conditions adapted to lessen the excess of reduced chromium.

2. The Erocess of treating a bath for depositing c romium electrolytically, said bath containing chromic acid and a compound recoverable upon analysis in the form of C508, and in which bath is employed an iron or steel anode, comprisin passing current through the bath in a dlrection opposite to that employed for de ositing chromium, the iron or steel anode omn cathode and another anode being used wit such relation t to the cathode as to oxidize the reduced' chromium.

In ttimony whereof I aiiix m si ature.

WILLIAM M. P IPS. 

